1. Field of the Invention
This invention relates to reservoirs which store hydraulic fluid for a working unit which depletes fluid from or replenishes fluid to the reservoir on a demand basis. In particular, the invention relates to such reservoirs which are vented.
2. Discussion of the Prior Art
Many types of hydraulic working units such as hand pumps typically include a reservoir for containing hydraulic fluid to be used by the pump. The pump depletes hydraulic fluid from or replenishes hydraulic fluid to the reservoir on an as-needed basis.
When hydraulic fluid is drawn from the reservoir, a vacuum is created in the reservoir. In some reservoirs, an air pocket is provided even when the reservoir is full to reduce the magnitude of the vacuum drawn. However, this is inefficient because it does not fully the reservoir volume for fluid and pumping efficiency is reduced because of the vacuum drawin in the reservoir.
Other reservoirs have been vented to the atmosphere so that air can be drawn in to replace the depleted fluid and prevent an excessive vacuum from occurring in the reservoir. However, providing a vent in the reservoir has limited the orientation that the pump can be used in, since if it is used in an orientation which causes fluid to flow to the vent, fluid can escape from the reservoir through the vent. Also, such systems which are vented to atmosphere (or have an air pocket in the reservoir) sometimes resulted in air being introduced into the pumping system, which could cause the pump to lose prime or make the system unstable. Vented systems also sometimes resulted in contamination of the hydraulic fluid, as would occur if a contaminant entered the reservoir through the vent.
Vented hydraulic reservoirs are also known in which a fluid chamber and a vented air chamber are separated by a sliding piston. In these systems, a sliding seal had to be provided by the piston between the air and fluid chambers, which resulted in leakage, a tolerance problem, erratic performance due to friction and sticking and many of the other problems associated with systems in which the hydraulic fluid was directly vented to the atmosphere.
Non-vented reservoirs in which the reservoir is a flexible bladder, for example, an expansible flexible bladder such as an elastomeric bladder, have also been employed to avoid venting the fluid in the reservoir directly. In these reservoirs, the hydraulic fluid fills the bladder, and the bladder expands or contracts in response to fluid being added to it or depleted from it. These systems are relatively difficult to fill and also sometimes provide a resistance to fluid being replenished to the bladder by the working unit.